The mechanisms for motor neuron degeneration in amyotrophic lateral sclerosis (ALS) are not understood. Our preliminary studies indicate that motor neuron death in ALS may occur by programmed cell death (PCD). We have identified abnormalities in proapoptotic and antiapoptotic protein levels in individuals with ALS, leading to our hypothesis that neurodegeneration in ALS may occur by PCD, involving subcellular redistributions of cell death proteins. Our studies also suggest that neuronal apoptosis in ALS may be Fas-mediated. The proposed experiments are designed to further evaluate this mechanism for motor neuron death. We will examine, in postmortem central nervous system tissues from individuals with ALS and from age and disease matched controls, the possible role of Fas-mediated PCD as a mechanism for neuronal apoptosis in ALS, by measuring the expression of apoptosis- inducing cell surface receptors and their ligands as well as the subcellular expression of apoptosis regulatory proteins, specifically members of the Bcl-2 and caspase families and caspase target proteins. We have also found that axotomy and target deprivation of motor neurons (by sciatic nerve avulsion) in adult rodents causes apoptosis. Motor neuron apoptosis in this model is associated with mitochondrial accumulation within the cell body and oxidative stress. We will use this model of apoptosis to test the hypothesis that motor neurons undergoing apoptosis in adult mouse spinal cord develop mitochondrial abnormalities, oxidative stress, and release cytochrome c which corresponds termporally with activation of caspases. We will identify whether the mechanisms for motor neuron apoptosis are dependent on Bax, p53, or Fas/Fas ligand by evaluating the progression of avulsion-induced motor neuron apoptosis in mice deficient in these genes. We will then use interventions, including antioxidant, caspase inhibitor and immunosuppressant/mitochondrial premeability transition blocker therapies, to prevent or delay motor neuron apoptosis in mice. These experiments will identify the contributions of PCD mechanisms to the pathogenesis of ALS and will clarify the molecular pathways leading to motor neuron apoptosis in vivo. These studies should lead to a better understanding of motor neuron death and to the design of new therapeutic experiments critical for the future treatment of ALS.